1. Field of the Invention
The present invention relates to an automatic brake control system which automatically applies a braking force to a driving vehicle according to the running velocity of the driving vehicle and a vehicle ahead which is running ahead of the driving vehicle and the distance therebetween.
2. Description of Related Art
Conventionally, many systems have been known that a radar unit thereof detects the distance between the driving vehicle and the obstacle or vehicle ahead and the relative velocity therebetween so that the system produces a warning and automatically makes a braking action for the driving vehicle when the driving vehicle is driven dangerously close to the obstacle or the vehicle ahead. For example, Japanese Patent Public Disclosure No.54-33444, laid open to the public in 1979, discloses a system which produces a warning when the actual distance between the driving vehicle and the obstacle ahead thereof becomes less than a value which is provided by adding predetermined distance to a quick braking distance determined by the relative distance and velocity of the two vehicles or the driving vehicle and obstacle wherein the braking force is quickly applied to the driving vehicle under the quick braking action when the actual distance becomes less than the quick braking distance. Further, Japanese Utility Model Public Disclosure No.64-83, laid open to the public in 1989, describes a system which calculates the warning distance for producing a warning based on the velocity of the driving vehicle and the relative distance and velocity between the driving vehicle and vehicle ahead, and which produces a warning, when the two vehicles get close to each other less than the warning distance, so that the sound of the warning changes by two steps sequentially.
A desired vehicle distance between the driving vehicle and the vehicle ahead which is necessary for the driving vehicle to safely avoid the obstacle or the vehicle ahead (the velocity of the driving vehicle V.sub.1 being greater than that of the vehicle ahead V.sub.2) can be calculated by subtracting the braking distance of the vehicle ahead from that of the driving vehicle at the quick braking action. Thus, the desired vehicle distance L.sub.1 is provided by the following equation. EQU L.sub.1 =(V.sub.1).sup.2 /2.alpha..sub.1 g-(V.sub.2).sup.2 /2.alpha..sub.2 g
Wherein .alpha..sub.1 g and .alpha..sub.2 g are the decelerations of the driving vehicle and the vehicle ahead respectively. As shown in FIG. 7, the desired vehicle distance L.sub.1 is needed to be increased, as a difference between V.sub.1 and V.sub.2 or a relative velocity V' between the driving vehicle and the vehicle ahead is increased.
A driver's braking vehicle distance L.sub.0 at which the driver actually starts the braking action taking account of the change of the vehicle distance is also shown in FIG. 7. When the relative distance and velocity are in the hatched portion (A) in FIG. 7, the driver actually makes the braking action. As seen from FIG. 7, the driver's braking vehicle distance L.sub.0 is set close to the desired vehicle distance L.sub.1 when the relative velocity V' is small. On the other hand, when the relative velocity V' is big or when the driving vehicle quickly gets close to the vehicle ahead, the driver's braking vehicle distance L.sub.0 is considerably greater than the desired vehicle distance L.sub.1. In the area where the relative velocity V' is comparatively big, if the warning distance is set close to the driver's braking vehicle distance L.sub.0, the warning is produced at the substantially same timing as the driver's braking operation. However, the warning would be undesirably produced because the warning distance is too far from the desired vehicle distance L.sub.1. On the other hand, if the warning distance is set too close to the desired vehicle distance L.sub.1, the driver feels uneasy so that the warning is useless because no substantial time is left for the driver to make a braking action after warning.